Conventional laser systems for digital projection typically consist of an array of blue lasers that are focused to a spot size of several mm2, such as 2 mm2, on a rotating phosphor wheel. The phosphor wheel is rotating at a typical speed of 60 Hz. A phosphor is partially coated on a rotating disk. The reflected light from the phosphor is used for the green channel of a projector. The rotating disc may comprise a transparent segment used to transmit blue laser light for the blue channel of the projector. The red channel of the projector may be made with a red LED module. Such a projector is known as a hybrid LED-laser projector, time-sequentially driven in connection with a DLP display.
With another example of a phosphor wheel the phosphor is used in a reflective mode and coated onto a metal substrate for improved cooling.
There also exist full laser phosphor projectors that do not contain LED sources. In this case the phosphor wheel is used in transmissive mode to generate white light that is subsequently divided into R, G and B with dichroic mirrors for use in LCD projection (with 3 LCD panels).
Solutions employing a phosphor wheel use the motion of the wheel to distribute the incident laser light and cool the phosphor. This has several drawbacks, including:                The system includes moving parts that may wear out and are sensitive to failure. When the rotation of the wheel fails, the cooling function is lost and the device will fail, posing a reliability and/or safety risk.        Moving parts cannot be allowed in all applications, for instance in applications with moving lamps, such as entertainment lamp/gobo projectors, to prevent gyroscopic effects.        The phosphor wheel increases the size of the system. A common wheel size has a diameter of 3-4 cm.        It is difficult to add extraction optics to the phosphor layer, limiting efficiency within a small spot size.        The phosphor is not restricted along the rotation direction which increases the spot size due to light spreading within the phosphor. This increases the etendue.        A phosphor wheel is an expensive component to make. A 100$ wheel cost is not uncommon.        Phosphor wheels are furthermore sensitive in reliability, prone to degradation. This is due to limited heat sinking capability and photo-thermal degradation sensitivity of the phosphor coated. An improvement can be made by using a ceramic phosphor on the phosphor wheel instead of a coating of a powder phosphor in a binder, but this is known to be a quite costly approach.        
As an alternative to the rotating phosphor wheel, static solutions have been proposed where laser(s) excite a phosphor layer, typically a ceramic phosphor attached to a heat sink body.
Such a solution is e.g. known from US 2013/0314893 A1 describing an illumination system comprising a laser light source and a wavelength conversion module for generating high brightness illumination by photoluminescence. The wavelength conversion module comprises an optical element comprising a wavelength conversion medium, the wavelength conversion medium being set in a mounting for heat dissipation, and an optical concentrator. The optical element comprises in a specific embodiment a length being equal to its diameter.
Furthermore, and especially in projection the source etendue is very important. The etendue is the product of the source area times the solid angle of the emitted light distribution. The etendue defines the usable light that is produced from the source when matched to the acceptance etendue of the display panel. Hence a too large source etendue may generate a lot of light, but this cannot be harvest by the optical system of the projector. A too small source etendue can put unnecessarily high thermal loads to the source and limit the optical efficiency. An example of a target etendue for a 0.67″ DLP display panel is a source etendue of 17 mm2 sr.
Thus, there is still a need for providing a static solution in which overheating of the wavelength conversion module is avoided and with which the reliability and efficiency is improved and the output luminous flux of the light emitting device is increased. Furthermore, there is a need for providing a static solution which is more versatile, simpler in construction, more robust and safe and which is more cost efficient to manufacture. Finally, there is a need for providing a static solution with a light output having a suitable etendue in view of the above considerations.